The vitreous (the gel that fills the back of the eye) becomes more of a liquid as you age. The reduced volume of the liquid pockets results in the vitreous eventually collapsing into itself. Flashes can be experienced at this point due to traction on the retina stimulating the photoreceptors.
The vitreous pulls off from its attachment to the optic disc, taking a fragment of tissue with it that then floats in front of the retina. Light hits the tissue fragment and it throws a shadow onto the retina, experienced as a black floater. Occasionally the pull of the vitreous is strong enough to tear the retina. If the tear is not treated, fluid can enter through the tear and under the retina, detaching it. If a retinal detachment is untreated blindness will result.
Although people often get used to floaters, they can be very symptomatic. Vision can be constantly interrupted which is distracting and frustrating.